There are many electronic systems for which it is highly undesirable to permit tampering with the circuitry. For instance, any device relating to the security of financial transactions is vulnerable if the circuitry can be tampered with. A bank card reader, for example, could thus be backed or otherwise compromised to bypass the security and used for criminal purposes.
Many such devices have therefore been created which involve a security circuit, a circuit which is easily broken when tampered with which permanently deactivates the device. Typically the security circuit will take the form of a flexible or deformable layer having conductive tracks on a lower surface which can be lain over the primary circuit. The conductive tracks can be actuated into electrical contact with the primary circuit, thereby completing the circuit. If the device is tampered with, the flexible layer will break, destroying the security circuit, and breaking the connections of the primary circuit.
In the case of a bank card reader, there is a security circuit which covers switching domes. When a keypad button is depressed, the security circuit deforms collapsing the switching dome, shorting together switch contacts on the PCB, located under the centre of the dome and around the outer edge of the dome.
Similar security domes are provided to electrically connect the security circuit to the primary circuit. These domes are held in the operated condition by movable spigots associated with the housing, when the housing is in the closed condition. Opening of the housing moves at least one of these spigots, releasing the associate dome, thereby triggering an alarm condition. This allows a transaction to progress, safe in the knowledge that no third party has interfered with the reader.
However, this system clearly only works if there are movable spigots present in the electronic device. It may be desirable to integrate security circuits into a great range of devices to prove that they have not been tampered with. To be effective, the security circuit must be in permanent electrical connection with the primary circuit.
Unfortunately, the main advantage associated with the spigot-based engagement means is that a strong connecting force is applied between the conductive tracks of the primary and security circuits upon moving the spigot, thereby ensuring that the circuit is completed successfully. However, it can be difficult to create sufficient force with the housing to close all security domes. A dome typically requires 100 to 150 grams of activation force, and pushing four to six security domes in a normal transaction device can often be too much for a standard housing to cope.